What common mistakes can lead to PHP code not functioning properly?

One common mistake that can lead to PHP code not functioning properly is using incorrect syntax, such as missing semicolons at the end of statements or mismatched parentheses. Another mistake is not properly declaring variables or using undefined variables in your code. Additionally, including a file with errors or typos can also cause issues with your PHP code. Example fix:

// Incorrect syntax: missing semicolon
$name = "John"
echo "Hello, $name!"; // Missing semicolon

// Corrected code
$name = "John";
echo "Hello, $name!";